The Black Hole Occupation Fraction as a Fossil Record of Seeding, Dynamics, and Galaxy Assembly

Abstract

The black hole occupation fraction (BHOF) is a powerful but intrinsically layered diagnostic of massive black hole (BH) seeding and subsequent galaxy assembly. We measure the total, central, and wandering BHOF in the ASTRID cosmological hydrodynamical simulation from $z=5$ to $z=0$ in galaxies with stellar masses ranging from $10^7$-$10^{12}$ $\mathrm{M}_\odot$. For the full population, the total BHOF remains close to unity across most stellar masses and redshifts, reflecting efficient seeding in eligible halos. At low stellar masses, the central BHOF declines toward the present day, while the wandering BHOF rises, indicating that mergers dynamically redistribute BHs away from galaxy centers. We further show that selecting only heavy-seed descendants substantially decreases the low-mass BHOF; meanwhile, selecting only active BHs yields a much smaller late-time BHOF at all stellar masses, demonstrating that AGN-selected samples trace duty cycles rather than intrinsic BH occupation. At $z=0$, primary (central) galaxies have higher BHOFs than satellites, and star-forming low-mass galaxies preferentially host wandering rather than central BHs. Our results show that the BHOF, when decomposed by BH location, seeding history, accretion state, and larger-scale galactic environment, encodes a rich fossil record of BH origins and dynamics.